Intermetallic layers and coatings are often formed on a surface of a metal component to protect the underlying metal substrate of the component and to extend its useful life during operation. For example, many superalloy components in gas turbine engines, like turbine blades, vanes, shrouds, and nozzle guides, include an aluminide coating on airflow or gas washed surfaces that protect the underlying superalloy base metal from high temperature oxidation and corrosion. Among other applications, gas turbine engines are used as aircraft or jet engines (e.g., turbofans), as industrial gas turbine engines for power generation, as part of mechanical drive units for items such as pumps and compressors, and as power plants providing motive forces to propel vehicles.
Generally, gas turbine engines include a compressor for compressing air, a combustor for mixing the compressed air with fuel, such as, but not limited to, jet fuel, natural gas, diesel, biomass waste gases, naptha and gasified coal gases, the mixture is subsequently ignited. The engine also includes a turbine blade assembly for producing power. In particular, gas turbine engines operate by drawing air into the front of the engine. The air is then compressed, mixed with fuel, and combusted. Hot exhaust gases from the combusted mixture pass through a turbine, which causes the turbine to spin and thereby powers the compressor. Aircraft gas turbine engines, referred to herein as jet engines, propel the attached aircraft forward in response to the thrust provided by the flow of the hot exhaust gases from the gas turbine engine. Rotation of the turbine in industrial gas turbine engines generates electrical power.
Air flow surfaces of certain turbine engine components are directly contacted by the hot exhaust gases. The hot exhaust gases heat these components to high temperatures and expose them to impurity elements like sulfur originating from the combusted fuel. Superalloys, in particular, are susceptible to severe oxidation and corrosion in such harsh environments, particularly when the superalloy components of the gas turbine engine are heated by the hot exhaust gas stream created in a jet engine.
Superalloy turbine engine components experience sulfidation when exposed at low temperatures to sulfur originating from the hot exhaust gases and other environmental sources. Generally, sulfidation increases the corrosion rate of superalloys and, in particular, the hot corrosion rate of nickel-based superalloys. Sulfidation is most often observed on portions of superalloy gas turbine components that are heated to temperatures below about 1500° F. (815° C.) during service. Often, superalloy gas turbine components are cooled by a stream of lower temperature air directed through a hollow interior region.
Sulfidation may occur on portions of superalloy gas turbine components that are shielded from direct exposure to the exhaust gas stream, but nevertheless operate at temperatures less than about 1500° F. (815° C.) and are exposed to sulfur from the hot exhaust gases that bypass sealing surfaces. For example, certain gas turbine blades include an airfoil segment that is heated to a temperature greater than 1500° F. (815° C.) when exposed to an exhaust gas stream, a root used to secure the gas turbine blade to a turbine disk of the gas turbine engine, and a platform that separates the airfoil segment from the root. In such gas turbine blades, the root, which is not directly exposed to the exhaust gas stream, is heated by conduction from the airfoil segment and also cooled to less than 1500° F. by heat transfer to the more massive turbine disk. The area of the gas turbine blade beneath the platform is particularly susceptible to sulfidation attack.
Aluminide coatings have been disfavored on certain surfaces of turbine engine components. Most aluminide coatings embrittle the surface of the superalloy material used to manufacture turbine engine components, which may cause a loss of surface ductility because the aluminide coating is not ductile.
Aluminide coatings may unwantedly alter the tight dimensional tolerances required on certain components. For example, areas below the platform, including the root of gas turbine blades, must maintain tight dimensional tolerances to properly couple the airfoil with the turbine disk. As a result, measures are routinely taken to avoid forming aluminide layers on machined pressure faces or root fixing surfaces below the platform when aluminiding the surfaces of the airfoil segment. Nevertheless, areas below the platform remain susceptible to corrosion enhanced by mechanisms like sulfidation.
Platinum aluminides have been proposed as a solution for averting sulfidation attack of regions of the superalloy turbine engine components below the platform. However, platinum aluminide coatings under certain operating conditions may be susceptible to cracking, which provides a path for the migration of sulfur and other corrosive elements to the unprotected superalloy surface. As a consequence of the ensuing sulfidation, the platinum aluminide coating may spall and delaminate, which is not acceptable during operation of the gas turbine engine.
Accordingly, there is a need for a coating effective to protect low-temperature surfaces of turbine engine components from corrosion damage.